Methods for transfer molding encapsulation of a semiconductor die with attached heat sink

ABSTRACT

A semiconductor device includes a heat sink adjacent to a die. A dam is positioned at the peripheral edges of the heat sink. During a transfer molding process, the dam serves two purposes. First, the dam prevents damage to the mold. Second, the dam prevents encapsulant packaging compound material from flowing onto the heat sink. The dam may be a gasket. The dam may also be a burr created by, for example, stamping the bottom of the heat sink. The dam may include copper, polyamides, and leadlock tape. The dam may be permanently connected to the heat sink or removed following packaging. The dam may be removed mechanically, through the use of heat, or during an electrolytic deflash cycle.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/287,665,filed Apr. 7, 1999, now U.S. Pat. No. 6,403,387, issued Jun. 11, 2002,which is a continuation of application Ser. No. 08/804,911, filed Feb.25, 1997, now U.S. Pat. No. 6,001,672, issued Dec. 14, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the transfer molding of semiconductordevices. More specifically, the present invention relates to a method ofusing a dam in transfer molding encapsulation of a semiconductor deviceand a heat sink, and in the resulting semiconductor device assembly.

2. State of the Art

A semiconductor integrated circuit (IC) device (referred to as a die orchip) includes bond pads on the active surface thereof for interfacingthe integrated circuits of the semiconductor device with other circuitsoutside the die located on differing substrates. Since the semiconductordevices are relatively small and the attendant bond pads on the activesurface thereof are, in comparison, considerably smaller, lead frameshaving a plurality of leads thereon connected to the bond pads of asemiconductor device are used to connect the semiconductor device withother circuits on differing substrates.

In a conventional lead frame design for use with an integrated circuitsemiconductor device, the lead frame includes a plurality of leadshaving their ends terminating adjacent a side or edge of the integratedcircuit semiconductor device with the device being supported by the diepaddle portion of the lead frame. Electrical connections are made bymeans of wire bonds extending between the leads of the lead frame andthe bond pads located on the active surface of the integrated circuitsemiconductor device.

Subsequent to the wire bonding operation, portions of the leads of thelead frame and the integrated circuit semiconductor device may beencapsulated in suitable plastic material to form a packagedsemiconductor device assembly. The leads and lead frame are then trimmedand formed to the desired configuration after the packaging of thesemiconductor device in the encapsulant material.

In a Leads-Over-Chip (LOC) type lead frame configuration for anintegrated circuit semiconductor (IC) device assembly, the leads of thelead frame extend over the active surface of the semiconductor devicebeing insulated therefrom by tape which is adhesively bonded to thesemiconductor device and the leads of the lead frame. Electricalconnections are made between the leads of the lead frame and bond padson the active surface of the semiconductor device by way of wire bondsextending therebetween. After wire bonding, the leads of the LOC leadframe and the semiconductor device are encapsulated in suitable plasticto encapsulate the semiconductor device and portions of the leads.Subsequently, the leads are trimmed and formed to the desiredconfiguration to complete the packaged semiconductor device.

By far the most common manner of forming a plastic package about asemiconductor device assembly is molding and, more specifically,transfer molding. In this process, with specific reference to an LOCtype semiconductor die assembly, a semiconductor die is suspended by itsactive surface from the underside of inner lead extensions of a leadframe (typically Cu or Alloy 42) by a tape, screen print or spin-ondielectric adhesive layer. The bond pads of the die and the inner leadends of the frame are then electrically connected by wire bonds(typically Au, although Al and other metal alloy wires have also beenemployed) by means known in the art. The resulting LOC die assembly,which may comprise the framework of a dual-in-line package (DIP),zig-zag in-line package (ZIP), small outline j-lead package (SOJ), quadflat pack (QFP), plastic leaded chip carrier (PLCC), surface mountdevice (SMD) or other plastic package configuration known in the art, isplaced in a mold cavity and encapsulated in a thermosetting polymerwhich, when heated, reacts irreversibly to form a highly cross-linkedmatrix no longer capable of being re-melted.

The thermosetting polymer generally is comprised of three majorcomponents: an epoxy resin, a hardener (including accelerators), and afiller material. Other additives such as flame retardants, mold releaseagents and colorants are also employed in relatively small amounts.While many variations of the three major components are known in theart, the focus of the present invention resides in the filler materialsemployed and their effects on the active die surface.

Filler materials are usually a form of fused silica, although othermaterials such as calcium carbonates, calcium silicates, talc, mica andclays have been employed for less rigorous applications. Powdered, fusedquartz is currently the primary filler used in encapsulants. Fillersprovide a number of advantages in comparison to unfilled encapsulants.For example, fillers reinforce the polymer and thus provide additionalpackage strength, enhance thermal conductivity of the package, provideenhanced resistance to thermal shock, and greatly reduce the cost of thepolymer in comparison to its unfilled state. Fillers also beneficiallyreduce the coefficient of thermal expansion (CTE) of the compositematerial by about fifty percent in comparison to the unfilled polymer,resulting in a CTE much closer to that of the silicon or galliumarsenide die. Filler materials, however, also present some recognizeddisadvantages, including increasing the stiffness of the plasticpackage, as well as the moisture permeability of the package.

When a heat sink is used on a semiconductor device assembly package,encapsulation of the semiconductor device becomes more difficult duringthe transfer molding process. In the first instance, the inclusion ofthe heat sink along with the semiconductor device attached to the leadframe makes the transfer molding of the assembly more difficult as morecomponents must be placed and aligned within the mold cavity.Misalignment of the semiconductor device and the heat sink within themold cavity may cause bleeding and flashing of the resin moldingcompound over the heat sink. Furthermore, when the heat sink, which isusually copper or an alloy thereof, rests against the mold surfaceduring the transfer molding process, damage to the mold surface canoccur by the mold surface being scratched and/or worn from contacttherewith by the heat sink. The resulting worn mold surfaces cause theresin molding compound to bleed and flash over the outside of the heatsink during the transfer molding process. This affects the ability ofthe heat sink to transfer heat to the surrounding environment during theoperation of the semiconductor device as well as presenting a poorappearance of the molded semiconductor device assembly.

Accordingly, there is a need for an improved transfer molding processfor packaging semiconductor devices having heat sinks associatedtherewith to help prevent or reduce the bleeding or flashing of themolding compound over portions of the heat sink during the transfermolding process of the semiconductor device assembly.

SUMMARY OF THE INVENTION

The present invention is directed to a semiconductor device assemblythat includes a heat sink adjacent to a die. A dam positioned about theperipheral edges of the heat sink during the transfer molding processserves to help prevent damage to the mold and help prevent encapsulantpackaging compound material from flowing onto the heat sink. The dam maybe a resilient, non-metallic material. The dam may also be a protrusioncreated by, for example, stamping the heat sink from a sheet of materialor stamping the bottom of the heat sink to form the dam thereon. The dammay include a suitable metal material, such as copper, copper alloys,etc., and a suitable non-metallic material, such as polyamides, andtape. The dam may be permanently connected to the heat sink or removedfollowing packaging. The dam may be removed with heat or during anelectrolytic deflash cycle or, if desired, mechanically. The inventionmay be employed in connection with various types of lead frameconfigurations or, when a lead frame is not used, with a baresemiconductor device of the semiconductor device assembly.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming that which is regarded as the present invention,the advantages of this invention can be more readily ascertained fromthe following description of the invention when read in conjunction withthe accompanying drawings in which:

FIG. 1 comprises a flow chart of an exemplary process sequence forplastic package molding;

FIGS. 2A and 2B are side schematic views of a typical transfer molding,showing pre-molding and post-molding encapsulant positions;

FIG. 3 shows a top schematic view of one side of a transfer mold ofFIGS. 2A and 2B, depicting encapsulant flow and venting of the primarymold runner and the mold cavities wherein the semiconductor deviceassemblies are contained;

FIGS. 4A, 4B, and 4C depict encapsulant flow scenarios for a moldcavity;

FIG. 5 is a side view of a semiconductor device including a heat sink, adam, a die, and packaging compound;

FIG. 6 is a bottom view of the device of FIG. 5;

FIG. 7 is a first embodiment of the device of FIG. 5, taken along lines3—3 of FIG. 6 in which the dam is a gasket;

FIG. 8 is a second embodiment of the device of FIG. 5, taken along lines4—4 of FIG. 6 in which the dam is a burr;

FIG. 9 is a side view of a mold and a die, lead frame, heat sink, anddam positioned therein.

DETAILED DESCRIPTION OF THE INVENTION

To more fully understand the present invention in the context of theprior art, a brief description of a transfer apparatus and method forforming a plastic package about an LOC die assembly is provided. Theterm “transfer” molding is descriptive of this process as the moldingcompound, once melted, is transferred under pressure to a plurality ofremotely-located mold cavities containing semiconductor deviceassemblies to be encapsulated.

FIG. 1 is a flow chart of a typical process sequence for plastic packagemolding. It should be noted that the solder dip/plate operation has beenshown as one step for brevity; normally, plating would occur prior totrim and form.

FIGS. 2A and 2B show pre-molding and post-molding positions ofencapsulant during a transfer molding operation using a typical moldapparatus comprising upper and lower mold halves 10 and 12, each moldhalf including a platen 14 or 16 with its associated chase 18 or 20.Heating elements 22 are employed in the platens to maintain an elevatedand relatively uniform temperature in the runners and mold cavitiesduring the molding operation.

FIG. 3 shows a top view of one side of the transfer mold apparatus ofFIGS. 2A and 2B. In the transfer mold apparatus shown, the encapsulantflows into each mold cavity 44 through the short end thereof.

In operation, a heated pellet of resin mold compound 30 is disposedbeneath ram or plunger 32 in pot 34. The plunger descends, melting thepellet and forcing the melted encapsulant down through sprue 36 and intoprimary runner 38, from and through the mold cavities 44 through theshort side thereof flowing across the semiconductor device assemblies100, wherein semiconductor device assemblies 100 comprisingsemiconductor devices 102 with attached lead frames 104 are disposed(usually in strips so that a strip of six lead frames, for example,would be cut and placed in and across the six cavities 44 shown in FIG.3). Air in the runners 38 and 40 and mold cavities 44 is vented to theatmosphere through vents 46 and 48. At the end of the molding operation,the encapsulant is “packed” by application of a high pressure toeliminate voids and reduce non-uniformities of the encapsulant in themold cavities 44. After molding, the encapsulated semiconductor deviceassemblies 100 are ejected from the cavities 44 by ejector pins 50,after which they are post-cured at an elevated temperature to completecross-linking of the resin, followed by other operations as known in theart and set forth in FIG. 1, by way of example. It will be appreciatedthat other transfer molding apparatus configurations, as well asvariations in the details of the described method are known in the art.However, none of such are pertinent to the invention, and so will not bediscussed herein.

Encapsulant flow in the mold cavities 44 is demonstrably non-uniform.The presence of the semiconductor device assembly 100 comprising asemiconductor device 102 with lead frame 104 disposed across themid-section of a cavity 44 splits the viscous encapsulant flow front 106into upper 108 and lower 110 components (FIG. 4A). Further, the presenceof the (relatively) large semiconductor device 102 with its relativelylower temperature in the middle of a cavity 44 permits the flow front106 on each side of the semiconductor device to advance ahead of thefront which passed over and under the semiconductor device 102.

FIGS. 4A and 4B show two mold cavity encapsulant flow scenarios where,respectively, the lower flow front 110 and the upper flow front 108 leadthe overall encapsulant flow front 106 in the cavity 44 containing thesemiconductor device assembly 100. FIG. 4C depicts the advance of a flowfront 106 from above, before and after a device 102 is encountered, theflow being depicted as time-separated instantaneous flow fronts 106 a,106 b, 106 c, 106 d, 106 e, and 106 f. As the encapsulant flow frontadvances and the mold operation is completed by packing the cavities,encapsulant pressure in substantially all portions of the cavitiesreaches hydrostatic pressure.

Referring to FIG. 5, a semiconductor device assembly 210 includes asemiconductor device 214 having bond pads 215 located thereoninterconnected to a lead frame 220 by one or more wire bonds 217 and aheat sink 216 adjacent to the semiconductor device 214. Thesemiconductor device 214 may be separated, if desired, from heat sink216 through a portion of the lead frame 220. Various types of lead framearrangements of the lead frame 220 may be employed, such as conventionaltype lead frames or Leads-Over-Chip (LOC) type lead frames, for example.Alternatively, a lead frame is not required with the semiconductordevice 214 being connected to the heat sink 216 and encapsulated, exceptfor the active surface of the semiconductor device 214 having bond pads215 thereon. After the encapsulation of the lead frame 220,semiconductor device 214, and heat sink 216 during the transfer moldingprocess, the encapsulant compound material 224 surrounds thesemiconductor device 214 and heat sink 216, except where prevented fromdoing so by the dam 228. A dam 228 is positioned at the peripheral edgesof heat sink 216 which prevents the flow of encapsulant molding materialfrom bleeding over or flashing around the heat sink 216 during theencapsulation of the lead frame 220, semiconductor device 214, and heatsink 216 in the molding process in the transfer molding apparatusdescribed hereinbefore.

FIG. 6 shows a bottom view of semiconductor device assembly 210. As seenin FIG. 6, dam 228 preferably extends substantially around theperipheral edges of the bottom of the heat sink 216. During a transfermolding process, dam 228 serves two purposes: (1) the dam 228 preventsdamage to the mold and (2) the dam 228 prevents encapsulant molding(packaging) compound material 224 from flowing (i.e., bleeding orflashing) onto heat sink 216.

Referring to FIG. 7, dam 228 may be a suitable resilient material or agasket formed of suitable material which is suitable for such use andwhich is suitable for use in the transfer molding process, such aspolyamides, Kapton™ tape, etc. The resilient material forming the dam228 may be applied to the periphery of the bottom of the heat sink 216by a molding operation, such as molding a suitable plastic material dam228 about the periphery of the heat sink 216.

Referring to FIG. 8, dam 228 may also be a protrusion or substantiallycontinuously formed burr type edge or lip extending around the peripheryof the bottom surface of the heat sink 216 created, for example, throughthe stamping of the heat sink 216 from a sheet of material or,alternately, by the stamping of the bottom of heat sink 216 to form theprotrusion thereon.

FIG. 9 shows a mold 234 to encase encapsulant compound material 224 thatis received through an encapsulant compound source 238 during thetransfer molding process as described hereinbefore.

Dam 228 prevents encapsulant compound material 224 from flashing orbleeding over the outside of heat sink 216. Flashing or bleeding occursdue to a combination of high pressure used in the molding process andthe inherent inconsistency in the flatness of the mold and the heatsink. When the compound bleeds over the heat sink, the heat sink becomesless effective due to a loss in exposed surface area of the heat sink.

Dam 228 may include any suitable metal, such as copper, aluminum, copperalloys, aluminum alloys, etc., polyamides, and leadlock tape. Leadlocktape may consist of a Kapton™ carrier film and have an adhesive coatingor material thereon. (See FIG. 7.) These materials are softer and wouldprobably form more of a gasket than would copper, thereby resulting inless resin bleed and a longer mold life. In such a case, the dammaterial would not have to have good adhesion to encapsulant compoundmaterial 224 or heat sink 216 since dam 228 only needs to be presentduring the molding process. Dam 228 may be permanently connected to theheat sink or removed following packaging. Dam 228 may be removed withheat or during an electrolytic deflash cycle.

As used herein, the term “adjacent” does not necessarily mean touching.For example, a heat sink may be adjacent to a die, although separatedfrom the die by a lead frame. Further, the term “connected” or a relatedterm does not necessarily mean directly connected but could includebeing indirectly connected.

Having thus described in detail preferred embodiments of the presentinvention, it is to be understood that the invention defined by theappended claims is not to be limited by particular details set forth inthe above description as many apparent variations thereof are possiblewithout departing from the spirit or scope thereof.

1. A method of packaging a semiconductor device assembly having asemiconductor die having a plurality of bond pads thereon and having aportion thereof attached to a portion of a lead frame having a pluralityof leads, at least one bond pad of the plurality of bond pads of thesemiconductor die connected to at least one lead of the plurality ofleads of the lead frame, comprising: forming a heat sink having a topsurface and a bottom surface, each top and bottom surface havingperipheral edges therearound; forming a substantially continuouslyformed protruding edge from the heat sink at the peripheral edges of thebottom surface of the heat sink; positioning the top surface of the heatsink adjacent to the semiconductor die having the plurality of bond padsthereon; placing the heat sink and semiconductor die in a transfermolding apparatus having the protruding edge formed at the peripheraledges of the bottom surface of the heat sink in contact with a portionof the transfer molding apparatus; and removing a dam from adjacent to asurface of the heat sink following an encapsulation of the semiconductordie and the heat sink.
 2. The method of claim 1, wherein the protrudingedge is removed with heat.
 3. The method of claim 1, wherein theprotruding edge is removed during an electrolytic deflash cycle.
 4. Amethod of packaging a semiconductor device assembly having asemiconductor die having a plurality of bond pads thereon and having aportion thereof attached to a portion of a lead frame having a pluralityof leads, at least one bond pad of the plurality of bond pads of thesemiconductor die connected to at least one lead of the plurality ofleads of the lead frame, comprising: forming a dam adjacent a peripheryof a heat sink from material of the heat sink, the dam protruding fromedge portions of the periphery of the heat sink; positioning the heatsink adjacent to a semiconductor die; positioning the dam, heat sink,and semiconductor die in a transfer molding apparatus having the damcontacting a portion of the transfer molding apparatus; encapsulatingthe semiconductor die and heat sink in molding material in the transfermolding apparatus; and removing the dam from adjacent to a surface ofthe heat sink following the encapsulation of the semiconductor die andthe heat sink in the molding material in the transfer molding apparatus.5. A method of packaging a semiconductor device assembly having asemiconductor die having a plurality of bond pads thereon and having aportion thereof attached to a portion of a lead frame having a pluralityof leads, at least one bond pad of the plurality of bond pads of thesemiconductor die connected to at least one lead of the plurality ofleads of the lead frame, comprising: providing a heat sink; forming adam adjacent a periphery of the heat sink from a material thereof, thedam configured as a protruding edge portion of the periphery of the heatsink; positioning the heat sink adjacent to the semiconductor die;positioning the lead frame, the semiconductor die, the heat sink, andthe dam in a transfer molding apparatus, the dam contacting a portion ofthe transfer molding apparatus; encapsulating a portion of the portionof the lead frame, the semiconductor die, and the heat sink in moldingmaterial in the transfer molding apparatus; and removing the dam fromadjacent to a surface of the heat sink following the encapsulation ofthe semiconductor die and the heat sink in the molding material in thetransfer molding apparatus.